which tool to clean/smooth old concrete

I've reclaimed an old cistern as storage space in my basement. I'd like to smooth the floor down a bit and give it a coat of paint.
The floor appears to be 4" of good solid concrete and what parts of it I've exposed seems to fairly smooth. But the inside of the cistern was lined with two courses of brick, a parge coat, and tar. The floor wasn't a concern during construction [1940's?] so it has lumps, bumps, gobs of mortar and tar in varying thicknesses all over it.
I have a Bosch demo hammer with a 1" chisel bit & that does a pretty good job of bringing up the old mortar, but it takes a soft hand to not start eating into the concrete.
I see a few different possibilities in the Bosch department-- a scaling chisel? http://www.toolfetch.com/tools/HS1503.html a bushing tool http://www.toolfetch.com/tools/HS1520.html asphalt cutter http://www.toolfetch.com/tools/HS1506.html [the last is 5" wide, but doesn't look very substantial]
I've never used any of these. Can anyone recommend or warn me away from them?
Or is there a better tool I could rent?
The floor is only 6x8 feet so I don't need a huge or powerful tool. I'm also not looking for a glass smooth finish, just something smooth enough to walk on and clean enough to take deck paint.
Thanks, Jim
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The bush head will chip into the top of the concrete. It is meant to take very controlled cuts off the top of the concrete. It would be my weapon of choice. It will not leave a smooth finish. Remove all high spots. lumps, and tar. Plan on capping with a thin coat of a good cementitious floor stone:
Vinyl Patch Hadley's Tamms
I would not paint it.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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An angle grinder with a diamond cup and the right touch can make it as smooth as glass.

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Rather than spend hours chipping it smooth, I'd lay a sacrificial floor over it. Some pressure-treated 2x6 laid flat, cover with whatever cheap 1/2 or 3/4 plywood you can find, paint to suit. If wet floor is a concern, make it in 2-3 panels you can lift out when needed.
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-snip-

I'm not complaining. . . but it didn't just 'appear'.<g>

I would have loved to do that but I didn't have the headroom. Why they poured a concrete ceiling, covered it with a few yards of sand, then poured a porch floor over that [on beams-- it doesn't touch the sand] is beyond me. But I end up with a 6'4" ceiling. [and a weird 18" space with 4" of concrete above and below]
Jim
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after removing high spots, use a concrete grinder
looks like a lawnmower, but it runs on 110 volt house current (so a long cord), uses 2 or 3 expendable abrasive stones about the size of chalkboard erasers, rents at the home improvement plantation stores
use a good breathing mask too

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<snip>
Hijacking here. Would this also work for me or do I need a different tool? I have a 26x10 slab that has over the years become the floor of a 3 season porch. It has been painted several times, some of the paint is worn off, others has scaled, some is still good. I would like to remove all of it, clean the floor and then do a good paint job on it.
Harry K
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After looking at my options & with a little help from the guy at Nations Rent, I went with the grinder. [a double head Edco - like this http://www.bledsoerentals.com/Inventory/Details/floorgrinders.html ]

He suggested the stripper attachment. It is a carbide cutter mounted in a block. Made to strip paint/tar/rubber/mastic etc. I bought a spare set of cutters for $20, but didn't need them. If the mortar had any toughness left to it I might have.

For sure-- though at least it had an outlet for my shop vac so the dust was held down. [I forgot to turn the vac on one time & got a look at the dust without it.
The only downside to this job was horsing the beast down-- then up-- a flight of stairs by myself. It left a 2" strip around the edge that I'll strip by hand, but the main job was done on less than an hour.
Thanks to all who replied-
[and to turnkey-- it looks to me like this is the ticket. I'd check the paint to see how thick & ask where you rent the machine if you should use the strip-serts or the grinding stones. The stones will leave you with a better finish, but if the paint is real thick they might clog.]
Jim.
[PS-- The machine was $55 a day--- plus $20 for the cutters. . . $7 tax, $4 'environmental fee' and $7.50 damage waiver-- total $93. Awful considering I only used it an hour--- but awesome considering I was going at it with a chisel that would have taken me a week and not done as good a job]
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